The migratory aptitudes of alkyl groups in the gas phase 1,2-Wittig rearrangement have been determined experimentally as follows. An anion Ph-C-(OR1)(OR2), on collisional activation, competitively rearranges to the two 1,2-Wittig ions PhC(R-1)(OR2)(O-) and PhC(R-2)(OR1)(O-) [R-1 and R-2 = alkyl and R-1 < R-2]. These two ions respectively eliminate (ROH)-O-2 and (ROH)-O-1. The smaller alkanol is eliminated preferentially, indicating that R-2 (the larger alkyl group) is migrating preferentially (observed tert-Bu > iso-Pr > Et > Me): a trend generally taken to indicate a radical reaction. However, a Hammett investigation of the relative losses of MeOH from R-C6H4- C-(OMe)(2) shows this loss decreases markedly as R becomes more electron withdrawing, an observation not consistent with a radical reaction. Ab initio calculations [at the CISD/6-311 + + G**//RHF land UHF)/6-311++G** levels of theory] have been used to construct potential surface maps for the model 1,2-Wittig systems -CH2OMe--> EtO-, and -CH2OEt-->PrO-. Each of these exothermic reactions involves migration of an alkyl anion. There are no discrete intermediates in the reaction pathways. There is no indication of a radical pathway for either rearrangement. It is proposed that the gas phase 1,2-Wittig rearrangement involves an anionic migration, and that it is not the barrier to the early saddle point but the Arrhenius A factor (or the frequency factor of the QET), which controls the rate of the rearrangement. Weak H-bonding between the alkyl anion and the oxygen of the neutral carbonyl species acts as a pivot in holding the molecular complex together during the migration process. This electrostatic interaction increases with an increase in the number of hydrogens able to H-bond to oxygen and with the number of equivalent ways this H-bonding can occur. The relative migratory aptitude of alkyl anions bound within these molecular complexes is tert-Bu- > iso-Pr- > Et- much greater than Me-, an order quite different from the migratory aptitudes of anions expected from thermodynamic considerations. This conclusion indicates that great care must be exercised in utilising thermodynamically derived migratory aptitudes to explain the course of a kinetically controlled reaction in the gas phase.

The major fragmentations of isomeric dimethylphenoxide anions upon collisional activation are losses of H . and Me .. Two minor processes involve the formation of m/z 79 (C6H7-) (-C2H2O) and m/z 77 (C6H5-) [-(Me . + CHO .)]. The former is only observed when (at least) one of the ortho carbons bears a hydrogen substituent, the latter only when there is a 4-methyl substituent. (C) 1997 John Wiley & Sons, Ltd.

Deprotonation of cis- and trans-4-methoxycyclohexanol by HO- in the ion source of a mass spectrometer yields the (M - H)(-) alkoxide ions exclusively. Both of these ions, on collisional activation, form MeO(-), MeO(-)(H2O) and eliminate MeOH. The loss of methanol forms the base peak of the spectrum, and the structure of this daughter anion is shown to be the alkoxide ion from cyclohex-3-enol for both isomers. Evidence (based on product ion, deuterium labelling and AM1 semiempirical computational studies) indicates that the loss of methanol from the trans isomer proceeds by an internal S(N)2 cyclisation of O- at the four position (through a 1,4-epoxycyclohexane species) followed by 3,4 elimination. A similar sequence may occur for the cis-isomer, but in this case the process is not as energetically favourable as that for the trans isomer.

There is significant correspondence between certain skeletal rearrangement processes of close-shell organic anions in the condensed and gas phases. Examples of such correspondence include the acyloin, acyl oxyacetate, anionic oxy Cope, anionic Wolff, benzilic acid, Dieckmann, Lossen, Smiles and Wittig rearrangements. In contrast, there are some rearrangements observed in the condensed phase, which are either minor or do not occur at all in the gas phase. The Favorskii, Tiemann and Carroll rearrangements fall into this category. Finally, there are some gas phase rearrangements which have no condensed phase analogy: for example the negative ion pinacol/pinacolone and Beckmann rearrangements. These, and related processes are discussed in this Review.

Deprotonated isocamphanones rearrange to deprotonated camphor in the gas phase, presumably via the intermediacy of a homoenolate anion.

The collision-induced mass spectra of the o-methyl and o-ethylphenoxide anions show loss of formaldehyde and acetaldehyde, respectively. These processes are rationalized in terms of a reversible rearrangement o-RC6H4O- half arrow right over half arrow left [(C6H4)-OR] (R = alkyl).

Deprotonated 1,2-diols and beta-methoxyhydrins often eliminate ROH (R = H, Me) on collisional activation in the gas phase. These losses do not involve a negative ion pinacol rearrangement in acyclic systems in which there are no conformational restraints on the relative positions of the reacting groups. In the case of beta-methoxyhydrins, labeling studies show that the product ion is formed by the losses of a methoxide ion and a proton from adjacent positions. The absence of a deuterium isotope effect for this process precludes the operation of an elimination process: we propose that the loss of methanol proceeds via an epoxide cyclization viz. Me2C(OMe)C(O-)(Me)2 --> (MeO-)-[GRAPHICS]CH2=C(Me)C(O-)(Me)2 + MeOH. In contrast, epoxide cyclization does not occur when the two oxygenated substituents cannot adopt an anti orientation. Thus deprotonated cis-2-methoxycyclohexanol loses methanol via a pinacol rearrangement, while loss of methanol from the trans isomer produces deprotonated cyclohex-2-en-1-ol, presumably by an epoxide mechanism.

[1,3] Sigmatropic rearrangements, e.g. R1R2C(O−)CHCH2 → R1C(O−)CHCH2R2 and/or R2C(O−)CHCH2R1, only occur under special circumstances in the gas phase. No rearrangements occurs when R1 and R2 are alkyl or phenyl; when either R group benzyl, minor rearrangement is observed. When the R groups comprise a cycloalkyl ring, [1,3] rearrangement occurs readily for small strained rings (e.g. cycloropyl and cyclobutyl) but not at all for cyclopentyl and cyclohexyl systems.

The gas-phase Smiles reaction of RC6H4O(CH2)nO-(n = 2 or 3) is an ipso rearrangement which is strongly influenced by the nature of the substituent R. Electron-withdrawing groups enhance the rearrangement. When the substituent R is halogen or MeO, and occupies the ortho position, ortho cyclization competes with the Smiles rearrangement.

Both radicals and (even-electron) neutrals are lost when even-electron organic negative ions are subjected ot collisional activation in the gas phase. Generally, radical loss is less pronounced and less diagnostic (of some structural feature) than the ubiquitous (even-electron) neutral loss. There are exceptions to this generalisation: these often fall into one of the following classifications, i.e. There is a further type of “radical” loss, which is a stepwise process involving loss of H and (R − H). This occurs in particular cases and only when R is alkyl(⩾Et). The reactions are diverse but they may all be rationalized as proceeding through five-membered ring intermediates. The loss of “C2H5” from an alkylcarboxylate anion is a suitable example:

The collisional activation mass spectra of deprotonated (acyloxy)acetates [R1CO2C-(R2)CO2R3] show a number of decompositions (e.g., losses of CO, C2O2, R3OH, and HCO2R3) that occur following the 1,2 anionic rearrangement R1CO2C-(R2)CO2R3 --> R1COC(R2)(O-)CO2R3. A similar gas-phase rearrangement is proposed for deprotonated alpha-acyloxyacetonitriles, i.e., RCO2C-(H)CN --> RCOCH(O-)CN.

Alpha-alkoxyvinyltrimethylsilanes deprotonate to form two species -CH = C(SiMe3)OR and CH2 = C(Si(Me)2CH2-)OR. The silylcarbanion undergoes the unusual rearrangement reaction CH2 = C(Si(Me3)2CH2-)OR -->. ((CH2CO)Me2RSiCH2-) Me2RSiCH2- + CH2CO. Loss of (R-H) occurs when R greater-than-or-equal-to ET; it is suggested that this is a reaction of the vinyl anion system.

Allyl vinyl ether is reported to undergo a facile Wittig rearrangement to yield penta-1,4-dien-3-ol under base- catalysed conditions in the condensed phase. In marked contrast, the Wittig rearrangement is not a major reaction in the gas phase. Instead, initial rearrangement occurs by a Claisen process and subsequent fragmentations involve some of the most complex interconversions yet proposed for negative ions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

AbstractUnsaturated ethers of type (I) deprotonate in the gas phase to form carbanions (II) which, when n = 1 and 2 and R: ‐Et or higher alkyl, undergo predominantly β‐proton transfer to form the E1cB intermediates CH2=CH‐CH2‐(CH2)n‐O‐CH2‐CH2‐, which eliminate ethene to yield the alkoxide CH2=CH‐CH2‐(CH2)n‐O‐.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The negative ion spectrum of MeOCONOCH2CHCH2 shows a major peak corresponding to the methylcarbonate anion MeOCO−2. Evidence is presented which suggests that this is formed by the successive rearrangement pathway The first step could involve either a 1,2 or 1,4 anionic rearrangement; the second step is a 1,2 anionic rearrangement.

AbstractReaction of benzil with HO− in the chemical ionization source of a triple‐analyser mass spectrometer forms a [benzl + HO−] ion. This ion, upon collisional activation, gives a mass spectrum identical to that obtained from deprotonated benzilic acid under the same experimental conditions.

The C7H7O- ions formed by deprotonation of benzyl alcohol, norbornadien-7-ol and quadricyclin-7-ol are discrete species which all fragment by competitive losses of H, H2, CH2O and C6H6 on collisional activation. The isomeric ion from norbornen-2-one behaves differently undergoing retro cleavage to form HC2O- and C5H5- as product ions. The three deprotonated cresols are also distinct species. Anisole is deprotonated (by amide ion) to form both PhOCH2- and (C6H4)-OMe, ions which equilibrate prior to elimination of formaldehyde.

AbstractThe use of collisional activation mass spectra and heavy atom (13C and 18O) labeling shows that the products (III) and (IV) from (I) are formed through Smiles intermediates (II).